Summary One aspect of this project continues to be the generation of maximally efficient expression vectors for the specific antigens. We have generated a set of optimized expression vectors for HIV and SIV. HIV vectors are developed for eventual human clinical trials. In parallel, SIV expression vectors are developed and studied in the most faithful model system for human AIDS, ie., infection of Rhesus macaques by SIV, a virus closely related to HIV, which causes very similar pathology to human AIDS. Our results show that optimized DNA expression vectors in the absence of any other form of vaccine boosting are able to protect rhesus macaque from high viremia after challenge with a highly pathogenic SIVmac251 challenge. We have developed sets of plasmids expressing the majority of HIV or SIV antigens. These plasmids have entered human clinical trials through our CRADA collaborators. They have also been used in several collaborative vaccine studies and were shown to provide excellent priming in DNA-prime-virus boost vaccinations. We have also proposed to use DNA vaccination for periodically boosting the immune response. This comes from the realization that DNA vaccination boosts existing immune responses with every application and does not have the problem of viral vectors, which focus immune response to the vaccine vector. To further improve vaccine efficiency we study the intrinsic properties of the different candidate antigens. We have shown that modulating the form, stability and cellular fate of the DNA produced antigens has profound effects on their immunogenicity and the type of response generated. We perform comparative studies to develop optimal forms of several antigens.